In English, there aren't single words to distinguish your older brother from your younger brother. You have to add "younger" or "older". In Japanese, there are separate words that contain the meaning of "younger" or "older".

Just like "father" and "mother", there are two terms for "older brother" that are used at different times. One term, あに, is used when talking about your own brother (but not to him). Let's have Clara give us an example:

The other term, おにいさん, is used when talking to your brother (or about someone else's brother). Note that below Clara says おにちゃん instead of おにいさん. This adds a feeling of familiarity.